This invention constitutes of a vertical drain to dewater saturated, compressible soils.
A structure, e.g. a building or a road brings a load on the ground. If the ground consists of saturated fine soil, clay, silt or organic soil, the load to start with will mainly be taken up by the pore water in the soil. The pore water pressure will increase and the pore water will start to flow. The water will find the shortest way to a draining stratum or layer. Underneath the loaded area the degree of saturation will decrease and simultaneously the pore over-pressure. Simplified, the stress of the load will finally be taken up by the soil particles. The flow of pore water will cease.
The decrease of pore water will be noted as a settlement of the ground surface. The settlement under a given area will roughly be equal to the volume of water squeezed out, divided with the surface area. The settlement of a structure foundated on 10-20 m of clay can often reach 1 m and more. Since fine soils have a low permeability the settlement can go on for a long time, 50-100 years, depending upon the geological conditions.
Through installation of vertical drains in the soil the time to reach a certain settlement can however be significantly shortend. The procedure is as follows. By means of a mandrel a number of drains are installed in the area of the structure. The mandrel protects the drain during the installation. When the desired depths is reached the mandrel is withdrawn, while the drain remains in the soil. After that the drains have been installed a surcharge, slightly heavier than the weight of the structure, is applied. When the estimated settlement for the structure has been reached, the surcharge is removed, and the structure can be erected. The drain spacing, often 1-2 m, is usually choosen so that the desired settlement will be reached within 1-2 years, depending upon the actual conditions.
As far as known the idea of vertical drainage originated with D. E. Moran in the U.S.A. in the 1920's. Moran proposed that vertical sand columns should be installed in the ground.
In the mid of 1930's Walter Kjellman, Sweden, proposed the likely first prefabricated band-shaped drain Swedish Pat. Nos. 102.311 and 121.887. It was made of cardboard, had a width of 100 mm and a thickness of 4 mm. In the longitudinal direction the drain had 6-10 channels through which the water could flow to the ground surface and/or to a drained bottom layer.
After the Second World War with the development of the plastic industri, a number of plastic drains came on the market. The majority have a rectangular cross section, but even drains with a cirkular section are available.
The time to achieve a given degree of consolidation with a given drain spacing and cross section on the drains can be calculated by means of a formula developed by for instance Kjellman. ##EQU1## t=time D=area influenced by one drain
d=drain diameter, equivalent diameter PA1 c.sub.h =coefficient of consolidation, a function of the horizontal permeability in the soil PA1 U=degree of consolidation in percent
An important measure of the effectiveness of a drain is the drain diameter (d), referred to a circular drain. For a rectangular drain the drain diameter will be the perimeter of the filter wrapping divided by .pi.. It can be seen that the capacity of the drain will increase with an increased perimeter.